1. Field of the Invention
The present invention relates to an antenna and a radio terminal having the antenna thereof.
2. Description of the Related Art
A radio terminal such as a portable telephone set and a portable information terminal have miniaturized. With the miniaturization of the radio terminal, a radio terminal having a folded-type case or the like has been proposed and put into practice.
To prevent a reduction in sensitivity of the antenna caused by the miniaturization, an approach to arrange a small piece of a magnetic body near by a case-side power-feeding unit of a planar reverse-F antenna and lower a resonant frequency has been proposed (refer to Kawano et al. “A Study on Miniaturization of Planer Inverted F Antenna utilizing Magneto-Dielectric Material (3)”, B-1-25, 2004, Electronic Information Communication Society Communication Society Convention). Thereby, an antenna length can be apparently long, so that the proposed approach contributes to the miniaturization of the antenna. As for other approach using the magnetic material, a configuration in which an unnecessary current is efficiently restricted is proposed (refer to Jpn. Pat. Appln. KOKAI Publication No. 2002-264255).
In the aforementioned configuration, it is presumed that the antenna is only a planar shape (or bar shape). Therefore, when the antenna is mounted at a curved part such as a corner part of the case of the radio terminal, or when it is mounted at a movable part of the folded-type radio terminal, because a property of the antenna changes, there is the fear of property deterioration that the resonant frequency becomes higher than a desired resonant frequency. Especially, in the case of a folded-type, the antenna is bent. Accordingly, there is a problem that the antenna length becomes apparently short, because bent antennas are capacity-coupled with each other. The same problem occurs when the antenna is mounted at the curved part of the case.
As described above, when mounting the antenna at the curved part of the case of the radio terminal, there is a problem that the folding of the antenna results in deterioration of a property, for instance, that the resonant frequency becomes higher than desired one caused by the folding the antenna. The deterioration of the property mainly results from the coupling among antenna elements, and it is expected to reduce change in antenna characteristics caused by such curvature of the antenna.